Current models of selective adaptation for vowels implicate acoustic or
auditory mechanisms rather than phonetic mechanisms. However, inconsistencies
have been reported for /i/, /(small capital eye)/, and /(cursive beta)/ [Morse
et al. (1976)] in that the middle vowel /(small capital eye)/ did not elicit
significant adaptation effects. These inconsistencies may reflect category
goodness effects [Miller et al. (1983); Samuel (1982)] in that adaptation is
more robust for tokens further from the category boundary. In the present study,
category goodness was determined precisely for /i/, /(small capital eye)/, and
/(cursive beta)/ by obtaining within-subject rating data. Two continua were then
constructed for each subject with these three vowels as the endpoints. Vowel
adaptation was then assessed for the two continua using each subject's three
best exemplars as the adapting stimuli. Despite the fact that each subject's
adapting stimulus was, by definition, the ``best'' exemplar of that category,
only /(small capital eye)/ and /(cursive beta)/ showed significant adaptation.
The absence of adaptation for /i/ may reflect its status as a point vowel,
suggesting that extreme vocal tract positions, and their acoustic correlates,
are remarkably stable. Research in progress on the point vowel /a/ will reveal
whether this hypothesis is correct. [Work supported by NSF SBR94-21064.]